Control of occupational exposure when working within a reactor containment building at power Matthew Lunn British Energy Generation Ltd. Sizewell B Power Station United Kingdom

Sizewell B Power Station 4 Loop, 2nd Generation Westinghouse PWR 1200 MW, single unit First criticality 31 Jan month refuelling cycles Currently in Cycle 7: –November 2003 to Mar 2005

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External radiological hazards Fission neutrons –Thermal & Intermediate energies –Fast neutrons (>25 keV); not measured directly by Albedo dosimeter “k-factor” –Prior knowledge of neutron spectra & Albedo k-factor essential –Higher k-factor = harder neutron spectra Activation & fission products – 16 O(n,p) 16 N;  emissions at 6.1 & 7.1 MeV – 58 Ni(n,p) 58 Co; 59 Co(n,  ) 60 Co; 134 Cs/ 137 Cs; –Crud still dominates radiation fields around certain components

Internal radiological hazards Surface contamination –Activation & fission products –Fixed & loose contamination <40 Bq/cm 2 Airborne radioactivity –Particulate alpha < Bq/m 3 –Particulate beta/gamma & radioiodine < 0.1 Bq/m 3 –HTO vapour ~10 to 60 kBq/m 3 Internal monitoring & bioassay? –Estimated doses <1mSv; no formal internal dose assessment required –(from static air sampling results & area occupancy)

100% power +21m (Head Cable Bridge) ~ 0.50 mSv/h (  ) ~ mSv/h (n) +6.5m (Ground Floor) ~0.01 to 0.03 mSv/h (  ) < to 0.20 mSv/h (n) Average n:  ratio <0.5 k-factor: 2.0 to m (LHSI Accumulators) <0.005 mSv/h (  ) <0.005 mSv/h (n) Average n:  ratio = 1 k-factor: 1.8 to m (Main Operating Floor) ~0.01 to 0.10 mSv/h (  ) ~0.02 to 0.50 mSv/h (n) Average n:  ratio ~ 5 k-factor: 4.2 to m (SG Steam Space) 0.02 to 0.15 mSv/h (  ) 0.08 to 3.00 mSv/h (n) Average n:  ratio ~10 k-factor: 5.1 to 5.5

Radiation beams Pipe & cable penetrations Bioshield gates  /n radiation beams –Very steep doserate gradients –Up to 20mSv/h per metre –Beams may not interact with personal dosimeters No “multi-badging” of workers Access to areas prevented using barriers 3 mSv/h 0.03 mSv/h 1 mSv/h

Justification of entries Triviality of dose –<0.05 man.mSv; no further justification or optimisation required Lower doserates at 100% power –Especially near RHR system Improved industrial safety Resource minimisation –Flatten resource peaks (esp. scaffolding) where demand > supply Improved outage mobilisation –Install temporary shielding to enable faster release of plant areas Prevent a reactor trip (scram) –Avoid unit loss & dose from a forced outage recovery

Personal dosimetry EPD –Doserate alarm; 500  Sv/h –Dose alarm; 100  Sv –Dose alarm ~ 50% lower than usual Albedo –Max. k-factor of 5.5 used for all assessments Direct-reading electronic neutron dosimeter unavailable So, staff told to assume….. –Total dose ~ 10x EPD on 21m & above –Total dose ~ 2x EPD on 14m & below

Pre-job briefing tools ALARA Brief for all entrants Plant Information Sheets –Photograph of equipment –Radiological survey data –Practical precautions (e.g. what side to stand) –Map showing location Item Location Plans –A2 drawings showing location of plant items –Overlaid with general radiological conditions –Used in briefing room

Scope of work at power Barrier Tape

Maintenance etc. doses Barrier Tape

RP Doses Barrier Tape

Conclusions Wide variation in practices around world Little OE or published data available during planning –Setting dose constraints was difficult…. –Difficulty in preventing increase in work scope Individual & collective doses remained low…. –c.f. national limits & company dose constraint of 10mSv Some jobs justified & optimised when worked at power –e.g. RHR scaffolding & lagging Some minor jobs not optimised when worked at power –e.g. Transmitter calibrations, corrosion surveys? Proposed dose constraints for future containment entries –1.5 mSv per annum –15 man.mSv per annum